Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes: > On 5/28/21 12:00 PM, Magnus Karlsson wrote: >> On Fri, May 28, 2021 at 11:52 AM Jesper Dangaard Brouer >> <brouer@xxxxxxxxxx> wrote: >>> On Fri, 28 May 2021 17:02:01 +0800 >>> Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> wrote: >>>> On Fri, 28 May 2021 10:55:58 +0200, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: >>>>> Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx> writes: >>>>> >>>>>> In xsk mode, users cannot use AF_PACKET(tcpdump) to observe the current >>>>>> rx/tx data packets. This feature is very important in many cases. So >>>>>> this patch allows AF_PACKET to obtain xsk packages. >>>>> >>>>> You can use xdpdump to dump the packets from the XDP program before it >>>>> gets redirected into the XSK: >>>>> https://github.com/xdp-project/xdp-tools/tree/master/xdp-dump >>>> >>>> Wow, this is a good idea. >>> >>> Yes, it is rather cool (credit to Eelco). Notice the extra info you >>> can capture from 'exit', like XDP return codes, if_index, rx_queue. >>> >>> The tool uses the perf ring-buffer to send/copy data to userspace. >>> This is actually surprisingly fast, but I still think AF_XDP will be >>> faster (but it usually 'steals' the packet). >>> >>> Another (crazy?) idea is to extend this (and xdpdump), is to leverage >>> Hangbin's recent XDP_REDIRECT extension e624d4ed4aa8 ("xdp: Extend >>> xdp_redirect_map with broadcast support"). We now have a >>> xdp_redirect_map flag BPF_F_BROADCAST, what if we create a >>> BPF_F_CLONE_PASS flag? >>> >>> The semantic meaning of BPF_F_CLONE_PASS flag is to copy/clone the >>> packet for the specified map target index (e.g AF_XDP map), but >>> afterwards it does like veth/cpumap and creates an SKB from the >>> xdp_frame (see __xdp_build_skb_from_frame()) and send to netstack. >>> (Feel free to kick me if this doesn't make any sense) >> >> This would be a smooth way to implement clone support for AF_XDP. If >> we had this and someone added AF_XDP support to libpcap, we could both >> capture AF_XDP traffic with tcpdump (using this clone functionality in >> the XDP program) and speed up tcpdump for dumping traffic destined for >> regular sockets. Would that solve your use case Xuan? Note that I have >> not looked into the BPF_F_CLONE_PASS code, so do not know at this >> point what it would take to support this for XSKMAPs. > > Recently also ended up with something similar for our XDP LB to record pcaps [0] ;) > My question is.. tcpdump doesn't really care where the packet data comes from, > so why not extending libpcap's Linux-related internals to either capture from > perf RB or BPF ringbuf rather than AF_PACKET sockets? Cloning is slow, and if > you need to end up creating an skb which is then cloned once again inside AF_PACKET > it's even worse. Just relying and reading out, say, perf RB you don't need any > clones at all. We discussed this when creating xdpdump and decided to keep it as a separate tool for the time being. I forget the details of the discussion, maybe Eelco remembers. Anyway, xdpdump does have a "pipe pcap to stdout" feature so you can do `xdpdump | tcpdump` and get the interactive output; and it will also save pcap information to disk, of course (using pcap-ng so it can also save metadata like XDP program name and return code). -Toke
